1. Field of the Invention
The present invention relates to a semiconductor switching circuit generating a pulsed power of high voltage and large current.
2. Description of the Related Art
For generating plasma, a pulse having an abruptly raising edge, a very high peak voltage of ten and several kilo-volts to several hundreds kilo-volts and an extremely large current such as several thousands amperes.
FIG. 1 a conceptual circuit diagram showing a basic structure of such a discharge circuit. A capacitor C constituting an electrostatic capacitance for storing energy is charged by means of a current limiting element not shown within a time interval of millisecond order up to a high voltage E0 of a high voltage supply source P. Then, a switching clement SW performing a ultra-high switching operation is closed to discharge through a conductor having a very low inductance L and a high voltage is applied to a discharging portion H
Heretofore, as the ultra high speed switching element SW, use has been made of a thyratron which is one of a vacuum tube and can operate under high voltage and large current. However, the thyratron has the following demerits.
(1) It could not operate at a high repetition frequency.
(2) Its operation is unstable due to miss-ignition.
(3) Its life time is short, its maintenance is cumbersome and requires high cost.
(4) Its construction is complicated due to heater circuit and gas controller.
(5) Due to jitter at turning-on operation, reliable operation could not attained.
Nowadays a semiconductor switching element has been well developed in accordance with the progress in the power electronics, and several kinds of semiconductor switching devices can operate at a high speed under high voltage and large current. Among these semiconductor switching devices, attention has been paid on a static induction thyristor (in general, referred to SI thyristor). However, the thyratron could not be replaced by the known SI thyristor, because a breakdown voltage of the SI thyristor is lower than that of the thyratron. Therefore, it has been proposed to use a series arrangement of a plurality of SI thyristors S-1, S-2 . . . S-n as shown in FIG. 2.
As stated above, in the known switching circuit for generating a pulsed power, it is necessary to use the DC voltage supply source P which could supply a high voltage, and therefore its size is liable to be large. Moreover, the capacitor C should have a high breakdown voltage, and thus it is liable to be large in size and expensive in cost. Furthermore, in case of using the series arrangement of a plurality of SI thyristors S-1, S-2 . . . S-n, the following problems occur.
Due to variation in characteristics, particularly in a leak current upon cutting-off an applied voltage of the semiconductor switching devices, voltages shared by respective semiconductor switching devices might be unbalanced. Then, there is a fear that excessive high voltage might be applied to one or more semiconductor switching devices. In order to mitigate such a drawback, it is necessary to select a number of semiconductor switching devices having identical characteristics. However, in practice, it is rather difficult to select semiconductor switching devices having identical characteristics.
As illustrated in FIG. 2, in order to mitigate or adjust unbalance between the semiconductor switching devices, i.e. SI thyristors S-1, S-2 . . . S-n in the series arrangement, balancer resistors R-1, R-2 . . . R-n are connected in parallel with respective SI thyristors. In this case, it is necessary to flow a current through the balancer resistors R-1, R-2 . . . R-n, said current being larger than a current flowing through the SI thyristors S-1, S-2 . . . S-n by about ten times. Therefore, very large energy loss occurs by the balancer resistors R-1, R-2 . . . R-n, and an efficiency of the circuit might be decreased to a large extent. There also occurs a problem of treating heat generated by the balancer resistors R-1, R-2 . . . R-n. Moreover, in the known circuit, there is a serious problem of unbalance of a transient voltage sharing upon a turn-on due to a variation in a stray capacitance of respective thyristors with respect to the ground.
If a trigger timing for respective thyristors S-1, S-2 . . . S-n fluctuates, an excessive high voltage is applied to a thyristors for which a trigger timing is delayed, and this thyristor is broken. Therefore, a trigger timing for all the thyristors must be coincided. However, it is quite difficult to coincide a trigger timing for respective thyristors S-1, S-2 . . . S-n. Furthermore, if one thyristor is erroneously made conductive, excessive voltage is applied to the remaining thyristors and they are destroyed.
As depicted in FIG. 2, gate driving circuits D-1, D-2 . . . D-n are provided for respective thyristors S-1, S-2 . . . S-n. Since a high voltage is shared by a number of thyristors, potentials of the gate driving circuits D-1, D-2 . . . D-n are different to a large extent. Therefore, power supply sources as well as gate driving signals of the gate driving circuits have to be isolated. In this case, dielectric strength amounts to several tens kilovolts, and therefore a circuit construction becomes very complicated and an operational reliability might be decreased.
Since a high voltage of several tens kilovolts is applied also to the series arrangement of a plurality of thyristors, an oil insulation should be provided between the series arrangement of thyristors and a component such as a chassis. However, this results in large labor word and cost. Therefore, in the known switching circuits, it is impossible to attain a complete isolation and a reliable operation could not be performed.
The present invention has for its object to provide a novel and useful switching circuit for generating pulsed power, in which the above mentioned various problems of the known switching circuit including the series arrangement of a number of semiconductor switching devices can be removed or at least mitigated, no excessively high voltage is not applied to the semiconductor switching devices ever under a variation of trigger timing thereof and the switching devices can be prevented from being destroyed, and no strict isolation is required between driving circuits for the semiconductor switching devices.
According to the invention, a switching circuit for generating pulsed power comprises:
first and second input terminals to be connected to a DC voltage supply source;
a plurality of capacitors whose one ends are connected commonly to said first input terminal and whose other ends are connected commonly to said second input terminal;
a plurality of magnetic cores;
a plurality of semiconductor switching devices each of which is connected in parallel with a respective one of the capacitors by means of a respective one of a plurality of primary conductors each being magnetically coupled with a respective one of said plurality of magnetic cores;
a plurality of driving circuits for controlling turn-on and turn-off of said plurality of semiconductor switching devices, respectively;
a series arrangement of a plurality of secondary conductors each being magnetically coupled with respective magnetic cores; and
first and second output terminals, both ends of said series arrangement of a plurality of secondary conductors being connected said first and second output terminals, respectively such that a sum of voltages induced in respective secondary conductors is applied across said first and second output terminals.
In the switching circuit for generating pulsed power according to the invention, a high voltage is not applied to each of the semiconductor switching devices, and therefore they can be effectively protected from breakdown although they do not have identical characteristics. Furthermore, since a same voltage is applied to all the semiconductor switching devices, no high voltage is applied to the driving circuits. Therefore, it is no more necessary to isolate power supply sources of the driving circuits as well as driving signals, and all the driving circuits may be energized by a same power supply source as well as a same driving signal. Moreover, the DC voltage supply source and capacitors are not necessarily formed by special ones for high voltage, and may be formed by conventional parts which can be available easily. In this manner, a cost of the switching circuit can be reduced.
In the switching circuit for generating pulsed power according to the invention, said each of said primary and secondary conductors is preferably wound on the magnetic core by one turn. Such a one-turn structure can be realized by simply passing the conductor through the magnetic core. In this case, said series arrangement of secondary conductors is preferably formed by a single conductor passing through said plurality of magnetic cores successively. Since the secondary conductor is subjected to a high voltage, it is formed by a rather thick and thus rigid conductor. Therefore, it is advantageous to construct the secondary circuit only by passing the conductor through the magnetic cores successively. In this case, the secondary conductor is preferably straight.
Furthermore, in the switching circuit for generating pulsed power according to the invention, it is preferable to connect a plurality of diodes connected in parallel with respective primary conductors in an opposite polarity to that in which a current flows through the semiconductor switching devices. When turn-on timings in the semiconductor switching devices fluctuate, a rather high reverse voltage is induced in a primary conductor by means of the secondary conductor and is applied to a semiconductor switching device. Then, a diode is made conductive and the reverse voltage is not applied to the semiconductor switching device. The same function can be attained by connecting one electrodes of the semiconductor switching devices commonly.
Furthermore, in the switching circuit for generating pulsed power according to the present invention, it is preferable that each of said semiconductor switching device is formed by a static induction thyristor.